The present invention relates to hydraulic control systems for automatic transmissions.
One of the conventionally proposed hydraulic control systems for automatic transmissions is known in JP-A 5-248526. This reference describes a low-clutch sequence valve for draining an accumulator back pressure of a low clutch on the release side in taking account of a rise of a working pressure of a band brake on the engagement side and the accumulator back pressure of the low clutch on the release side.
With the known hydraulic control system, however, a drain timing is determined based on the accumulator back pressure of the low clutch on the release side, so that if the accumulator back pressure is varied, its drain timing is shifted. Specifically, the accumulator back pressure on the release side and the working pressure on the engagement side produced from a line pressure or a source pressure are varied together, so that if the accumulator back pressure is reduced due to dispersion or deterioration of a device, a working pressure which serves to drain the accumulator back pressure is also reduced, resulting in lowered switching pressure in a characteristic of the working pressure on the engagement side. This decreases the torque capacity on the release side though a sufficient torque capacity on the engagement side is not secured yet, causing engine racing.
It is, therefore, an object of the present invention to provide hydraulic control systems for automatic transmissions which always ensure achievement of an optimum timing for draining the accumulator back pressure on the release side upon shifting regardless of variations in hydraulic pressure due to dispersion or deterioration of a device on the release side.